1. Field of the Invention
The present invention relates to chlorinated ethylene copolymers with superior rubbery properties. More particularly, it is concerned with chlorinated ethylene/nonconjugated diene copolymers which are capable of forming rubbery vulcanized product with superior properties by vulcanization.
2. Description of the Prior Art
It is well known that ethylene/.alpha.-olefin/nonconjugated diene copolymer rubber forms a rubbery material superior in properties such as ozone resistance and heat resistance by vulcanization. Use of said copolymer rubber is, however, limited because of its poor oil resistance, adhesivity and covulcanizability. It is also known that the above-mentioned poor properties can be improved by chlorinating said copolymer rubber (see Japanese Patent Publication No. 911/1966, Japanese Patent Publication No. 4829/1971). The chlorinated rubber as mentioned above, however, is unsatisfactory in that the strength is very low whether it is vulcanized or unvulcanized. In this respect, chlorinated rubber with a chlorine content of about 5-50% by weight and a Mooney viscosity (ML 1+4, 100.degree. C.) of about 20-150 produced by chlorination of ethylene/1-butene/5-ethylidene-2-norbornene copolymer rubber having an ethylene/1-butene molar ratio of about 85/15-95/5, for example, has been proposed as being superior in strength characteristics (see U.S. Pat. No. 4,433,105). However, the above-mentioned chlorinated rubber is still unsatisfactory in rubbery properties and needs improvement also in commercial production. In fact, use of a soft copolymer rubber is reasonable as the starting material in the production of soft chlorinated rubber, and the above-mentioned terpolymer with a specific ethylene/1-butene molar ratio meets the above requirement. In the chlorination reaction by the aqueous suspension method which is commercially advantageous, however, there is a problem that the DSC peak temperature of the starting copolymer is too low. The chlorination reaction temperature in the above prior art is considered to be about 90.degree.-70.degree. C. Our investigations, however, have revealed that the chlorination at such a low temperature is apt to result in heterogenous addition of chlorine to give a hard chlorinated proudct as well as leads to a tendency of deteriorating the thermal stability. Whereas oil resistance of chlorinated rubber is the highest in the products with a chlorine content of 35-40% by weight, chlorinated products of the above-mentioned terpolymer rubber which have a chlorine content within the above range are harder in quality and lower in thermal stability due to cohesive energy of chlorine.
In order to produce chlorinated ethylene copolymers superior in rubbery properties as well as in oil resistance, it is necessary to employ a chlorination temperature of at lowest 100.degree. C. which requires use of a starting ethylene copolymer having a peak temperature of at lowest 100.degree. C. according to the DSC method.